One example of a field rotating type synchronous generator has been disclosed in Japanese Patent Application Publication No. 2367/1958. This generator comprises a stator having an output winding connected to a load and main voltage building up means including a capacitor exciting winding and a capacitor connected across the ends of the capacitor exciting winding, and a rotor having a field winding across which a rectifier is connected. In such a generator, residual magnetism in a rotor core causes a voltage to be induced across the capacitor exciting winding, and accordingly, a current of advanced phase flows through the capacitor exciting winding. Increased magnetic action by the advanced phase current causes an occurrence of self-excitation while armature reaction of the capacitor exciting winding causes an electromotive force of reverse phase to be produced in the field winding. The electromotive force causes a field current to flow through the rectifier from the field winding. Thus, the field core is more excited to build up an output voltage across the output winding and the capacitor exciting winding. The load current through the load and the exciting current through the capacitor exciting winding increase the field electromotive force of reverse phase through their composite armature reaction, which causes the field current through the field winding to be enhanced. Therefore, in addition to the advantage of its brushless type generator, this generator has the advantages that decreasing a voltage drop of the output winding by the load can be restrained and that variation in the voltage can be also restrained. However, it is impossible that the prior art generator assures the rapid build-up of the voltage, because the reverse phase electromotive force is induced in the field winding only by the armature reaction through the current in the capacitor exciting winding. In order to assure the rapid build-up of the voltage across the output winding, there should be used core material having less loss of the magnetomotive force, which causes the generator to be expensive. Also, in order to increase the voltage induced across the capacitor exciting winding by residual magnetism in the core, there should be increased the number of turns of the capacitor exciting winding and the value of the capacitor, which causes the generator to be large-scaled.